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Selection Guide for Micro Self-Tapping Screws
I. Specification Parameters: Precisely Matching Base Material and Assembly Requirements
The specification marking of micro self-tapping screws follows the standard system of "head type + thread type + diameter × thread pitch × length", and each parameter directly affects the assembly effect. The selection of diameter must be based on the thickness of the base material as the core criterion: For PCB boards with a thickness of 0.8-1.5mm, screws of M1-M1.6 specifications are preferred. For example, a micro self-tapping screw of M1.2-0.25×3mm can not only avoid penetrating the substrate and damaging the circuit but also ensure the engagement depth. If the base material is an aluminum alloy shell with a thickness of 2-3mm, the specification should be upgraded to M2-M2.5 to prevent insufficient connection strength due to excessively small diameter.
The selection of thread pitch and lead must balance fastening efficiency and stability. The thread pitch of micro self-tapping screws is usually between 0.2-0.7mm. Single-thread screws (e.g., M2-0.4) are suitable for static fastening scenarios, such as fixing electronic device casings, as they have strong self-locking ability to prevent loosening. Double-thread screws (e.g., M3-0.6×2) have a larger lead, and their screwing speed is 50% faster than that of single-thread screws. They are applicable to scenarios requiring rapid assembly, such as automotive interior buckles, but need to be used with lock washers. The length parameter must follow the principle of "effective engagement length ≥ 1.5 times the diameter". For example, the effective engagement length of an M2 screw should be no less than 3mm. For countersunk head screws, the head thickness must be included in the total length to avoid fastening failure due to insufficient embedding.
Head type and slot type are related to tool compatibility and space adaptability. Pan heads (Type P) and countersunk heads (Type CS) are commonly used inside electronic devices: Pan-head screws (e.g., T3-P-M1.6) have a raised head, which facilitates disassembly and maintenance, making them suitable for connecting repairable modules. Countersunk-head screws (e.g., T2-CS-M1.4) are flush with the base material surface after being tightened, making them suitable for scenarios with high flatness requirements, such as camera modules. For the slot type, the Torx (internal 梅花) structure is preferred, as its anti-cam-out ability is more than 3 times higher than that of cross slots, enabling it to meet the high-torque requirements of automated assembly.
II. Material Selection: Balancing Strength and Environmental Adaptability
Material selection must comprehensively consider strength requirements, corrosive environment, and lightweight needs. Carbon steel offers the highest cost-effectiveness: Low-carbon steel (C1008) is suitable for 4.8-grade general fastening scenarios, such as fixing plastic casings. Medium-carbon steel (C1035) can reach 8.8-grade strength after heat treatment and can be used for connecting small metal components, but it needs surface treatment for rust prevention. Stainless steel is the first choice for high-corrosion scenarios: 304 stainless steel can withstand neutral salt spray for up to 48 hours, making it suitable for fixing sensors in humid environments. 316 stainless steel has 50% higher corrosion resistance and is often used in medical equipment and marine instruments.
Aluminum alloy materials focus on lightweight, with a density only 1/3 that of carbon steel. They are suitable for weight-sensitive products such as drones and wearable devices, but their strength is relatively low (equivalent to only 4.8-grade carbon steel), so they need to be used with larger-diameter specifications. Copper alloys combine electrical conductivity and corrosion resistance: Brass screws can be used for grounding connections of circuit boards, and beryllium copper materials can also meet elastic requirements, making them suitable for fixing contacts in vibrating environments.
Strength grade matching is a key supplement to material selection. Grade 4.8 screws are suitable for low-strength base materials such as plastic and wood. High-strength screws of Grade 8.8 or above need quenching and tempering treatment and can be used for metal-to-metal connections, but they may easily cause cracking in plastic base materials, so their use requires caution.
III. Surface Treatment: Balancing Rust Prevention and Assembly Characteristics
Surface treatment directly affects the service life and assembly stability of screws. Electrogalvanizing is a basic option: Blue-white zinc has a bright appearance and is suitable for exposed decorative parts; yellow zinc can withstand salt spray tests for up to 72 hours and is applicable to humid scenarios such as automotive interiors. However, electrogalvanizing has a hydrogen embrittlement risk, so it should be avoided for high-strength screws of Grade 10.9 or above. Phosphating treatment can form a lubricating film with a friction coefficient as low as 0.05, which improves torque consistency. It is the preferred choice for automated assembly, has no hydrogen embrittlement risk, and is suitable for high-strength screws.
Dacromet coating has much higher corrosion resistance than electroplating, withstanding neutral salt spray tests for more than 500 hours. It is suitable for outdoor equipment and industrial instruments, but has poor electrical conductivity and cannot be used for grounding connections, and its cost is 2-3 times higher than that of galvanizing. Black oxide treatment has the lowest cost, only forming an oxide film with a thickness of 0.8 microns, and its rust prevention ability is limited. It is only suitable for non-critical parts in dry indoor environments.
Special scenarios require customized treatment: For screws that need electrical conductivity in electronic devices, silver plating or gold plating can be adopted; for screws in high-temperature environments, high-temperature-resistant coatings should be selected to prevent coating peeling from affecting insulation performance.
IV. Scenario Adaptation: Deriving Selection Plans from Requirements
The consumer electronics field pursues miniaturization and efficient assembly. It is recommended to use 304 stainless steel micro self-tapping screws of M1.2-M2 specifications, combined with phosphating treatment and Torx slot type. For example, the T2-P-M1.4-0.3×4mm model can meet the rapid connection between PCB boards and plastic casings. The automotive industry focuses on vibration resistance and rust prevention: For the engine compartment, Grade 8.8 carbon steel screws with Dacromet coating (e.g., M3-0.5×6mm) are suitable; for interior components, galvanized aluminum alloy screws can be used to reduce weight.
Medical equipment has strict requirements for cleanliness and corrosion resistance, so it must use 316 stainless steel materials, and the surface should be passivated to remove oil stains and avoid drug contamination. For industrial instruments, adjustments should be made according to the environment: Dacromet coating is selected for humid environments, high-temperature-resistant phosphating treatment for high-temperature environments, and micro self-tapping screws with anti-loosening structures for vibrating environments.
After selection, double verification is required: Tensile tests to ensure the pull-out force meets standards (e.g., the pull-out force of M2 screws should be no less than 500N), and salt spray tests to match the rust prevention requirements of the environment, so that the final plan can be determined.